THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.

WOLF BLITZER, CNN HOST: Thanks very much. Happening now, the condition of the surviving Boston bombing suspect has been upgraded and a source says he's telling investigators who masterminded the attack.

The brutal killing of one of Tamerlan Tsarnaev's best friends, unsolved since 2011, is now being reviewed with a, quote, "wider group of eyes."

And we'll show you a controlled explosion of a pressure cooker bomb, as experts work to learn all they can about these cheap and deadly devices.

I'm Wolf Blitzer in Boston.

And you're in THE SITUATION ROOM.

Here in Boston, investigators are learning more about the suspects in last week's bloody massacres. Here are the latest developments.

A government official tells CNN the surviving suspect told investigators the wars in Afghanistan and Iraq were motivating factors in the attacks.

A New Hampshire fireworks store confirms that Tamerlan Tsarnaev bought two reloadable mortar kits with 48 shells. But law enforcement officials say it wasn't enough for explosions of the size that occurred in Boston.

An 8-year-old bombing victim, Martin Richard, was buried today after a private funeral mass. His parents issued a statement saying -- and I'll read it to you -- "We laid our son Martin to rest and he is now at peace."

Just minutes ago, the sisters of the bombing suspects, Ailina and Bella Tsarnaev, issued a statement through their attorneys. And it reads as follows. I'll read it to you. "Our heart goes out to the victims of last week's bombing. It saddened us to see so many innocent people hurt after such a callous act. As a family," the statement adds, "we are absolutely devastated by the sense of loss and sorrow that has caused. We don't have any answers, but we look forward to a thorough investigation and hope to learn more." And then they finally ask, "We ask the media to respect our privacy during this difficult time."

The surviving Boston bombing suspect, Dzokhar Tsarnaev, is improving in a local hospital. That comes a day after a federal judge found him to be alert, mentally competent and lucid at a bedside hearing.

CNN's Brooke Baldwin is joining us now from Beth Israel Deaconess Medical Center here in Boston -- Brooke, what do we know about the suspect's health?

BROOKE BALDWIN, CNN CORRESPONDENT: Wolf, we now know from the FBI and folks here at Beth Israel Hospital here in Boston that 19- year-old Dzokhar Tsarnaev has been upgraded from serious condition now to fair condition.

What does that mean?

Well, if you listen to our chief medical correspondent, Sanjay Gupta, he basically explains that that means that Tsarnaev's vital signs have stabilized, things like heart pressure -- blood pressure and your heart rate, for example. We know that he had an operation on Saturday, so a couple days out, his condition has been improving, which obviously is good news. That, of course, is predicated upon whether or not he decides to continue cooperating with investigators, but also, that his medical condition clearly helps in them gleaning some information out of him with regard to what happened last Monday.

But to quote Sanjay Gupta, he is now officially out of the woods -- Wolf.

BLITZER: Brooke, you also had a chance today to speak to some veterans who are trying to help some of these survivors.

What did they tell you?

BALDWIN: Yes, I tell you, Wolf, these are amazing stories. You know, you first hear the stories of the first responders who were running in to help some of these victims and now the stories of some wounded warriors. I talked to these two guys today. I spent my morning at Boston Medical, another great hospital here in the city of Boston. And one suffered in -- through an IED blast in Afghanistan in the Argan Valley a couple of years ago. He lost part of his leg. And another gentleman, Steve Chamberlain, from Florida, came up here because this mother/daughter duo that's recovering here in one room, they wanted to come help him, come talk them through what life will be like without limbs.

Here he was.

(BEGIN VIDEO CLIP)

BALDWIN: You lost your leg in an IED accident.

How do you try to explain to these people who are still reeling that it's going to be OK? STAFF SGT. TANNER KUTH (RET.), U.S. ARMY: Well, you know, you can only -- you can only tell them what you've been through. And, you know, I remember when I was in there, in Walter Reid. You know, there were amputees there and they were giving me words of encouragement and stuff like that. And it did help. But at the same time, you know, she's going to go through her own healing process and there's going to be times when no matter what we can tell her or see what we do, until she experiences it for herself, you know, she's going to have some self-doubt at some point.

But, you know, seeing what other people can do and seeing what we do with our lives as amputees, you know, it will give her that hope. And when she gets down, she'll be able to lean on that and go but if they can do it, eventually I'll do it. And she will.

(END VIDEO CLIP)

BALDWIN: So some optimism from these veteran amputees today.

I should mention, Wolf, that they were at Boston Medical visiting, as I mentioned, the mother/daughter duo. This was Celeste and Sydney Corcoran (ph). And today is Sydney's 18th birthday. She initially suffered what they thought would be a fatal wounding from last Monday's marathon attack. But the two of them, it sounds like, will be OK -- Wolf.

BLITZER: All right, Brooke.

Thanks very much.

Brooke Baldwin reporting from the hospital.

Meantime, the FBI is very interested in speaking with the widow of Dzokhar Tsarnaev's brother, Tamerlan Tsarnaev, to see if there is any way she might help. Her attorney says, she is, in his words, doing everything she can to assist with the investigation.

CNN's Chris Lawrence is joining us from Rhode Island, her hometown there, with this part of the story.

What's going on over there -- Chris?

CHRIS LAWRENCE, CNN CORRESPONDENT: Well, Wolf, Katherine Russell is back at home now after heading out with her lawyer for several hours earlier today. Her attorneys say that they have been speaking with federal investigators on her behalf, but won't say whether she has spoken to investigators directly.

(BEGIN VIDEOTAPE)

LAWRENCE (voice-over): Tamerlan Tsarnaev's widow hustled out of her parents' Rhode Island home Tuesday. Investigators want her help as they piece together the alleged Boston bomber's plan.

MIRIAM WEIZENBAUM, KATIE RUSSELL'S ATTORNEY: The reports of involvement by her husband and brother-in-law came as an absolute shock to them all.

LAWRENCE: Her attorney says Katherine Russell lived with Tamerlan in a cramped Cambridge apartment. As authorities try to determine when and where he may have assembled the bombs, investigators want to find out what, if anything, she knows.

AMATO DELUCA KATIE RUSSELL'S ATTORNEY: She is doing everything she can to assist in the ongoing investigation.

LAWRENCE: Russell's attorneys say she didn't know anything. They say she last saw Tamerlan before she went to work Thursday, before the FBI released this video. They say she worked as a home health aide, while Tamerlan stayed home with the couplet's young daughter.

AMOS TROUT PAINE, RUSSELL'S FORMER TEACHER: Very outgoing and friendly, very smart and very talented.

LAWRENCE: That's the Katie Russell Amos Trout Paine remembers. Her high school art teacher says she talked a lot about earning her college degree.

(on camera): Are you surprised how her life has turned out so far?

TROUT PAINE: I was surprised to find out that she had dropped out. And I hadn't seen any indication of a particular interest in a lot of religion.

LAWRENCE (voice-over): Russell was raised Christian in suburban Providence. She moved to Boston for college, met Tamerlan and dropped out. Attorneys say she converted to Islam and was an observant Muslim who wore the hijab, or head scarf.

(END VIDEO TAPE)

LAWRENCE: And sources close to the family say that Katie Russell didn't speak Russian, so she didn't always understand what was being spoken -- what was being said around the house. The attorneys say she was out of that apartment for long stretches of time because she could be working sometimes six to seven days a week, sometimes up to 70 hours -- Wolf.

BLITZER: Chris Lawrence in Rhode Island for us.

Thanks, Chris, very much.

Authorities, meanwhile, are now taking a fresh look at the slaying of one of Tamerlan Tsarnaev's best friends, unsolved since September, 2011. Three people were killed then in Waltham, Massachusetts. All had their throats slashed.

Let's bring in our national correspondent, Deborah Feyerick.

She's working the story for us.

What are you learning about this -- Deb?

DEBORAH FEYERICK, CNN NATIONAL CORRESPONDENT: Well, Wolf, we can tell you, a source is telling us that a number of agents are now investigating the marathon bombing. They've now turned their attention to a grisly triple homicide that was committed about 18 months ago in Waltham, Massachusetts. That murder initially appeared to be drug-related. And the reason they're looking at it is because one of the victims was a very close friend of bomb suspect, Tamerlan Tsarnaev.

Now, Brendan Mess and Tsarnaev were sparring partners. They trained together at a mixed martial arts center in Boston. Brendan Mess was brutally murdered in September of 2011, along with two other men. And investigators at the time said the heads of the three victims were pulled back and their throats slit ear to ear with great force.

Now, marijuana was spread over the bodies, according to a source, in what's described as a symbolic gesture. Thousands of dollars in cash were left behind at the crime scene.

The district attorney originally stated that it was not a random crime. The victims appeared to know their killers. And that's killers plural, suggesting that it would have taken more than one person to overpower the men. Remember, Brendan Mess was a mixed martial arts expert. A source says that Tamerlan Tsarnaev is believed to be one of the last people to have seen his best friend. A source says there's no indication that Tamerlan was ever interviewed by police at all, even to see whether Mess had any enemies.

Well, Tamerlan left America three months later, traveling to Russia, where he stayed for six months before returning to Boston.

The Middlesex County D.A.'s office says that they will, quote, "review any new information that comes to light," unquote -- Wolf.

BLITZER: Is it just a coincidence, according to investigators, or are they suspicious about the killing that occurred, what, on the tenth anniversary, specifically on 9/11?

FEYERICK: Yes, and it occurred just at that time. And that's what's fascinating, also, is that, you know, they can't understand why Tsarnaev would never have been questioned in connection, because that's the first thing you do, you question people who knew the victim. So that's one of the things that they are looking into, but also the connection -- the connection that you would have something, again, we get -- we've been saying this for a while now, you have something that simply doesn't make sense. And so you've got teams of investigators, each of whom are running down parts of Tamerlan Tsarnaev's life, just to see how they fit together. And this is one of those things, the fact that his best friend would be brutally murdered in the way he was killed and then Tamerlan leave the country just a couple of months later.

So they are looking at that closely.

BLITZER: Yes. I'm sure they're looking at all aspects of that mystery, as well.

Deb Feyerick, thank you.

The mother of the Boston bombing suspects is now speaking out. You're going to hear what she says about her sons. That's coming up.

And you'll also hear from the vice chairman of the Senate Intelligence Committee. Senator Saxby Chambliss is joining us to talk about the bombing investigation. He's just left the hearing where the FBI, behind closed doors, has briefed the senators.

(COMMERCIAL BREAK)

BLITZER: These are new pictures we're getting showing the marathon makeshift memorials that were left near the scene of the bombings, being moved into a park, which is still behind police barricades as of now. We're told that the park is expected to open in the next couple of days.

One of her sons is dead, the other lying wounded in a hospital, accused of a horrific bombing. But the mother -- the mother of these Tsarnaev brothers insists her sons are innocent.

(BEGIN VIDEO CLIP)

ZUBEIDAT TSARNAEV, BOMBING SUSPECT'S MOTHER: What happened is a terrible thing, but I know that my kids have nothing to do with this. I know it. I am mother. I have -- you know, I know my kids. I know my kids. Really, my kids would never get involved into anything like that.

(END VIDEO CLIP)

BLITZER: And what of the scrutiny of her elder son received from the FBI? Listen to what she says.

(BEGIN VIDEO CLIP)

TSARNAEV: They were monitoring him and I know that because I used to talk to them. They used to come to our house and like two, three times, and then my -- my Tamerlan used to tell me that he used to talk to them, too, because they called me once and they wanted his number and then, well at such moments, I used to get really worried because, you know, it is my kids and I his mother again.

So, and then when the agent called me back he said that, you know, I have spoken to your son and really got to understand that he's a really nice boy, you know, very intelligent, very like nice boy. It's just, do you think, he asked me, do you think that your Tamerlan could get involved into some kind of -- because he's like a leader, you know, he's a strong boy -- and do you think that he could get involved into kind of like any organization, you know, like radical organization probably he was meaning.

And, at that time, because they told me that they were, they controlled -- they saw whatever he was reading. So I said, no, no. (END VIDEO CLIP)

BLITZER: CNN by the way also caught up with the mother earlier today. That report coming up in our next hour. You'll be anxious to hear what else this mother has to say.

Meanwhile, officials are telling CNN that the surviving bombing suspect is giving new details about why these two brothers carried out the attacks. Joining us now, Republican senator, Saxby Chambliss of Georgia. He's the vice chairman of the Senate Intelligence Committee. They just wrapped up a closed door briefing with the FBI.

Senator, thanks very much for joining us. Apparently, this younger brother in the hospital is suggesting that the wars in Afghanistan and Iraq were motivating factors for these brothers to go ahead and plant those bombs at the Boston marathon. Is that what you're hearing as well?

SEN. SAXBY CHAMBLISS, (R) INTELLIGENCE VICE CHAIRMAN: Well, Wolf, i can't comment obviously on what the individual has said. The FBI will be giving what information they can about that, but the fact of the matter is that we've got, obviously, all of the elements that indicates the involvement of these individuals. As for the why of it, that's going to be a very complex and comprehensive investigation.

The FBI is going to be talking to not just these individuals, this individual, but other individuals who were acquainted with these two men to see what they can find out about their motive because I'll be honest, it is not clear even after the interview of the suspect in custody has been conducted, it's still not clear exactly why they did this.

BLITZER: Can you tell us what you've learned about what the Tsarnaev -- this 19-year-old has been saying at his bed side to investigators?

CHAMBLISS: Well, they've been asking the obvious questions and our interrogators are very skilled. I can't get into the details of exactly what he has said, but I think it's pretty obvious the questions that they have asked. And he's responded with mixed statements as well as mixed emotions about what took place and what did happen and the involvement of he and his brother. There are lots of inconsistencies that the FBI is going to have to ferret out.

BLITZER: What does that mean, mixed emotions?

CHAMBLISS: Well, obviously, you have a young man who's coming and going with respect to the sedation that he's been under and very traumatic experience for any 19-year-old and he's obviously shown some emotion about his involvement and the facts leading up to this taking place. We know that from other statements, from other people that he went back out in the community after last Monday and socialized with friends.

I think that tells you something about this individual and some of the emotions that we might be seeing. BLITZER: Are you saying that he's showing remorse at the hospital in the questioning or he's not or he's not showing any remorse? I wonder if you can elaborate on that.

CHAMBLISS: Yes. I don't think there's been any indication of remorse.

BLITZER: Has he bragged about what he and his brother did?

CHAMBLISS: I haven't been told that there's been any bragging on his part about the incident either, Wolf. It's been matter of fact, but you can't talk about what was done without showing some emotion.

BLITZER: Did he indicate that his older brother who's now dead actually masterminded the attack and that no international terrorist group was directly involved?

CHAMBLISS: That may or may not be the case. Obviously, we know that the brother became more and more radicalized over the last several years. We know that the brother traveled to Russia for about six months, the first part of 2012. He was in an area where lots of bad guys stay and train. Do we know that he was involved in any training there? The answer, clearly, is no at this point.

But those are the types of things, Wolf, that the FBI is spending time on today. They're doing a very thorough investigation all over the world with anybody who had any contact with these two individuals to try to see if they can't determine if anybody else was involved, were they in the United States, were they outside the United States. Those are unanswered questions at this point in time.

BLITZER: Is there evidence that the brothers were radicalized through their preachings of Anwar al-Awlaki, the American born cleric who was killed in a U.S. drone strike and specifically that "Inspire" magazine that has been circulating out there on the internet?

CHAMBLISS: There is a very clear indication that they reviewed "Inspire." As far as the influence of Awlaki, again, I have not heard that at this point. That's one of the things that I'm sure the FBI is going to be looking very closely into with respect to their websites, their e-mails, those types of things. But "Inspire" magazine is available over the internet and it's pretty clear that they read that magazine and whether that's where they learned how to make the bombs.

Again, we don't know the answer to, but, certainly, there are instructions in that journal that would tell you how to make bombs that are very similar to this. Not exactly like this but similar to the ones that were used.

BLITZER: I know you just emerged from a briefing with the FBI, a closed door briefing. Did the FBI drop the ball a couple years ago when the Russian authorities notified the U.S. that Tamerlan Tsarnaev was associating with Islamic radicals? Was there a blunder there?

CHAMBLISS: I think the FBI did a very thorough investigation of Tamerlan when they were notified by a foreign intelligence service, and they went back and asked for additional information not once, not twice, but three times, and they got no further information on exactly why that agency thought this individual might be radicalized. The FBI did the usual things you would expect as far as investigating the individual and it was a pretty thorough check.

And everything turned up negative. They went to websites. They went to friends of the individuals. They went to their parents. So, I think the FBI did a pretty thorough investigation. Were there mistakes made? Wolf, you will remember, I was very much involved in the post investigation of 9/11. We talked a lot then about sharing of information and trying to break down some stove pipes that existed within the intelligence and the law enforcement communities.

Well, we may be seeing some of those stove pipes re-erected here and we don't know that for sure, but that's a part of our oversight authority and our oversight duty. And we're trying to make sure that all of that information that was available was shared. If it wasn't, then there may be somebody who dropped the ball, but that's not evident at this point in time.

BLITZER: I know you and your colleagues on the Senate Intelligence Committee are going to do a thorough job looking back not for recriminations necessarily, but to make sure we all learn from what happened to avoid these kinds of potential mistakes down the road. Sen. Saxby Chambliss, thanks very much for joining us. We appreciate it.

CHAMBLISS: Thanks, Wolf. Good to be with you.

BLITZER: Up next, she was a dancer before she lost a foot in the Boston bombing, and she's vowing to dance again. Our own Anderson Cooper, he'll have the emotional story.

(BEGIN VIDEO CLIP)

ADRIANNE HASLET-DAVIS, INJURED IN BOSTON MARATHON BOMBINGS: I was right in front of it, right in front of the business where it was. So, I felt the direct impact and it immediately blew off my left foot.

(COMMERCIAL BREAK)

BLITZER: Welcome back. I'm Wolf Blitzer in Boston. Happening now, a dance instructor wounded in the Boston blast vows to dance again and teach our own Anderson Cooper to dance with her even though she no longer has a foot.

A shockwave that can be felt from a quarter mile away. CNN shows you up close just how deadly a pressure cooker bomb can be in the hands of a terrorist.

And the man accused of sending ricin tainted letters to President Obama and other U.S. officials has just been released from federal custody. We're standing by for a live news conference for explanation of what's been going on.

I'm Wolf Blitzer in Boston and this is a special edition of the SITUATION ROOM.

(BEGIN VIDEO CLIP)

(END VIDEO CLIP)

BLITZER: She was a dancer before the blast here in Boston, and she vows to dance again even though she no longer has her foot. CNN's Anderson Cooper visited this bombing victim in the hospital along with her husband who was also injured, and they spoke about her incredibly positive attitude. Watch this.

(BEGIN VIDEO CLIP)

ANDERSON COOPER, CNN ANCHOR: I hate to ask you to relive what you went through, so if you don't want to that's totally fine.

HASLET-DAVIS: It's OK. I relive it every day.

COOPER: You think about it every day.

HASLET-DAVIS: I do. Absolutely.

COOPER: How close were you to the second explosion?

HASLET-DAVIS: I was right in front of it. Right in front of the business where it was, so I felt the direct impact. And it immediately blew off my left foot.

COOPER: How far away was the bomb, do you know?

UNIDENTIFIED MALE: My guess would have been about five feet.

COOPER: Five feet.

HASLET-DAVIS: Yes. We're lucky to be alive.

COOPER: Did you lose consciousness?

HASLET-DAVIS: I don't think we did. I remember everything, so if we did it was for a matter of seconds.

COOPER: You remember being blown through the air.

HASLET-DAVIS: Yes.

COOPER: What happened then?

HASLET-DAVIS: I landed and sort of closed my eyes and was underneath Adam and kind of covering my head and face. It was very gray and quiet. Gray smoke and ashes, and a lot of debris falling. And I remember telling Adam oh, my gosh I'm alive and he said, I'm okay. I'm okay. Are you okay? Oh, my gosh. Are you okay? I said I think we're okay, and couldn't believe it. I couldn't believe that we survived and we weren't hurt at all.

And I didn't feel any pain. I had no idea what was -- what had happened. And then I sat up and I tried -- he said we've got to get out of here or something like that. I sat up and tried to move and I said oh, my gosh. My foot. There is something wrong with my foot. He lifted up my leg and we just lost it. Adam had a lot of blood all over his pants. It was hard to tell his injuries and my injuries because of the amount of blood everywhere. But I know he was hurt as well.

COOPER: What did you do then when you realized what happened to your foot?

HASLET-DAVIS: It just -- went into survival mode. I went into, I've got to do something about this. I can't lose my foot.

COOPER: That was in your mind.

HASLET-DAVIS: Yes. I can't lose my foot. I've got to get it somewhere safe and clean and I've got to get something wrapped around it. And then I grabbed the door open with my elbow and crawled in to Forum, dragging blood and asked a couple of people for help and finally received it. Got a couple people to do a tourniquet. And then Adam was shortly behind me. I wanted to stay with him, but my brain was just on I have to get to somewhere clean. I'm losing so much blood.

COOPER: When did you realize you didn't have a foot?

HASLET-DAVIS: I was - I woke up and my parents were there, and I hugged and kissed them and I said, Mom, can you help me? I feel like my foot is falling asleep because it feels like my ankle is falling off of the pillow and my foot is half on. And I realized that now that was phantom pain because she looked at me and said, Adrianne, you don't have a foot. Your foot is gone. And I just lost it. It was really hard to hear.

COOPER: You are determined to dance again.

HASLET-DAVIS: I am. Yes.

COOPER: Dancing is really important to you.

HASLET-DAVIS: It is so important to me. It's my life.

COOPER: What about it?

HASLET-DAVIS: Dancing is the one thing that I do - and I've said this many times - that dancing is the one thing I do that when I do it, I don't feel like I should be doing anything else -- ever. I feel so free and so wonderful. I'm big on music, and I feel like all of us when we hear music, we kind of move to the music and I feel like --

But I feel like it's just such a freeing thing. It's such a big part of my life and part of my life, also, is being able to teach people how to do that. So not only is it big in my life for me and an outlet for me -- I get to share that and see it developing in other people.

COOPER: You want to run the race next year.

HASLET-DAVIS: I do. I can't believe I said that. Adam's been making fun of me the whole time. He's like, I can't believe you said that. You're not a runner at all.

COOPER: You're not a runner.

HASLET-DAVIS: I'm not a runner at all. No. But I wasn't a ballroom dancer at one point in my life, either.

Something happens in your life where it's not exactly what you expected, you have to conquer that and you have to find the better side of it. It's not something that, I don't know. I don't want that to be the end. I don't want this to be the end. I'm only 32. I don't want this to be the end. So whether it is, you know, running the marathon or walking the marathon or crawling the marathon and being the last one across, I'm okay with that. I didn't say I'd win it. But I am defiant, and I want to come out stronger.

(END VIDEOTAPE)

BLITZER: What a wonderful, wonderful woman and her husband. We of course wish her only, only the best. Much more coming up from here in Boston including the charges the surviving bombing suspect now faces. We've just received some new information. We'll have that with you when we come back.

Also, a stunning twist in the case of a man accused of sending poison-laced letters to the president of the United States and other U.S. officials. New information on that development as well.

(COMMERCIAL BREAK)

ANNOUNCER: This is CNN breaking news.

BLITZER: We're getting some breaking news in THE SITUATION ROOM about whether or not the state of Massachusetts will file some charges in the Boston bombings. CNN's Ashleigh Banfield has been working her sources here and elsewhere. She is joining us on the phone right now. Ashleigh, what do you know?

ASHLEIGH BANFIELD, CNN CORRESPONDENT (on the phone): Wolf, working the sources and getting right to the top of the chain. I just came from the DA of Suffolk County who handles Boston, Dan Connolly and asked him pointedly what is the plan for the investigation and any filing of state charges in the killings of these victims of the bombing and the MIT officer?

And he was quite definitive, Wolf. He said the state will not be pursuing any of the charges at this point. They are feeding all of the criminal investigation and the charging of it to the federal authorities at this point, citing there are such robust statutes that are narrow enough and carry with them satisfactory penalties that there isn't the need for the redundancy on the state level of resources. Not only financial resources but man power.

That said, they are sharing their resources. They are feeding any requests that are needed for their local resources as well. But at this point, they do reserve the right to reactivate and bring in kind of any state-level charges if there is a need. And I asked what that need could be, and the DA said to me, say for instance there was a co- conspirator who was discovered, weapons charges of some kind. We could reactivate and get invested in this once again. And we could charge locally, but at this point, as far as murder goes, certainly they are feeding it to the federal authorities.

BLITZER: All right, Ashleigh, thanks very much. Ashleigh with the very, very latest from authorities here in Massachusetts.

Just ahead, the man accused of sending a poison-laced letter to President Obama suddenly released. We're learning, about to learn why. We're standing by for a live news conference on that event.

And ahead as well we'll show you a controlled explosion of a pressure cooker bomb as experts work to learn all they can about these cheap and deadly devices.

(COMMERCIAL BREAK)

BLITZER: A shocking twist in the case of the Mississippi man accused of sending President Obama and other U.S. officials ricin- tainted letters. His attorney now saying he has been released from custody. We're awaiting a live news conference. We'll have live coverage of that, but first CNN White House correspondent Dan Lothian is joining us with the latest. What do we know right now?

DAN LOTHIAN, CNN WHITE HOUSE CORRESPONDENT: Wolf, this is a stunning development in the case where the government insisted the suspect posed a threat to the public. Well, now he is out on bond as we wait to learn what move prosecutors will make next.

(BEGIN VIDEOTAPE)

LOTHIAN (voice-over): Was this Mississippi man accused of sending a threatening letter to the president, a senator, and a judge set up? His lawyer thinks so.

CHRISTI MCCOY, DEFENSE ATTORNEY: I do believe that someone who was familiar and is familiar with Kevin just simply took his personal information and did this to him. Kevin Curtis is absolutely 100 percent innocent.

LOTHIAN: And late signs that the case against the man the government considered a threat to the public may be falling apart. A preliminary hearing for 45-year-old Paul Curtis was abruptly canceled with no explanation. A short time later he's released on bond.

Curtis and his legal team have maintained his innocence from the start.

HAL NEILSON, DEFENSE ATTORNEY: At this point the evidence is lacking. It is not there. Any time you have information over the Internet or Facebook and you've got millions of people that can access that and that's what they have. There's just no evidence.

LOTHIAN: A law enforcement source tells CNN they are looking into whether someone tried to implicate Curtis in the suspicious letters.

Curtis is known around his community in Corinth, Mississippi, as an Elvis impersonator. In fact the senator he's accused of sending a threatening letter to says he hired him for a party a decade ago.

SEN. ROGER WICKER (R), MISSISSIPPI: He entertained at a party that my wife and I helped give for a young couple that was getting married. It was quite entertaining.

(END VIDEOTAPE)

LOTHIAN: So at this point everything is on hold. His attorneys who plan to hold a press conference, as you pointed out, Wolf, in the next hour say that the charges have not been dropped. The case has not been dismissed but that they feel better about their case today than they did yesterday -- Wolf.

BLITZER: What a -- what a mystery unfolding there. We'll of course have live coverage of that news conference. Lots of interest. These ricin-laced letters sent to the president and the senator who is behind it. Are there others out there? We'll have live coverage. You see the microphones already set up in Mississippi.

Up next, we're going to show you in slow motion and up close just how powerful a pressure cooker bomb can be.

(COMMERCIAL BREAK)

BLITZER: A pressure cooker bomb like the devastating devices here in Boston certainly can be built for under $100. CNN commissioned experts at a testing facility in New Mexico to build and detonate one. The idea is to learn how these devices work and possibly how to prevent them from doing so much harm.

Here's CNN's David Mattingly.

(BEGIN VIDEOTAPE)

DAVID MATTINGLY, CNN NATIONAL CORRESPONDENT (voice-over): At this remote desert testing ground, experts from New Mexico tech replicate and explode bombs used by terrorists. On this day, there's a sense of urgency.

(On camera): After Boston, what are you worried about? Could this be the future of domestic terrorism?

VAN ROMERO, NEW MEXICO INSTITUTE OF MINING AND TECHNOLOGY: Well, you're always worried about copycats. More and more people are they going to be using this.

MATTINGLY (voice-over): This is a pressure cooker bomb, similar to the bombs in Boston, and we're about to set it off.

(On camera): Are you going to do a countdown?

(Voice-over): In the wrong hands we already know how deadly this bomb can be, and we're not taking any chances.

(On camera): For safety reasons we've had to retreat to this mountain top here. We are now over a quarter of a mile away from where we left that pressure cooker.

(Voice-over): But that's still not far enough to avoid flying shrapnel. So we're watching from inside a bunker.

UNIDENTIFIED MALE: Five, four, three, two, one.

MATTINGLY (on camera): Wow. That white smoke looks just like what we saw in Boston.

ROMERO: Yes.

MATTINGLY: I could feel it all the way up here.

ROMERO: Oh, yes. That shock wave will travel all the way.

MATTINGLY (voice-over): But down below is the real shock.

(On camera): At this point we're looking for fragments.

(Voice-over): One bomb turned into thousands of weapons, scattered more than a hundred yards. This was part of the pressure cooker, now mangled and razor sharp.

(On camera): No wonder so many people got hurt.

(Voice-over): Instead of nails, we filled the pot with nuts from a hardware show. Shot out like bullets they pierced plywood, some even melted from the heat.

(On camera): Look at the back of it. How fast were these things moving when they went out of here?

ROMERO: They can travel 1,000, 2,000 feet a second.

MATTINGLY: A second? That's faster than sound. ROMERO: Right. They'll move faster than the speed of sound. These things will actually get in front of the shockwave and hit you before the shock -- the pressure wave does.

MATTINGLY: You're hit before you even hear it.

ROMERO: That's right.

MATTINGLY (voice-over): Here's what the blast looks like using a high-speed camera. An intense ball of fire, less than 20 feet across. But watch the white rings on the desert floor. That's the shockwave. Engineers studying this blast say there's a lesson in here for first responders.

(On camera): Let's say I'm a first responder. What do I need to beware of when I come up on a scene like this?

ROMERO: Well, there's a lot of shrapnel around, it's very hot, it's very sharp. You could easily cut yourself. There could be unexploded ordnance, parts of the bomb that are still left over that didn't explode when it was supposed to explode. That could go off at any time.

MATTINGLY: But for potential bystanders out of this demonstration, there are only words of caution. By the time you hear the boom, you could already be hit. Awareness of your surroundings could be the only defense.

(END VIDEOTAPE)

MATTINGLY: What we learned from this is that these devices are crude, they're cheaply made, and they're made for two reasons only, Wolf, and that is to spread terror and to kill.

BLITZER: To kill and maim.

MATTINGLY: That's right.

BLITZER: As many people as possible.

MATTINGLY: That's right. That device that you saw, because it was on the ground just like the one here in Boston, when that shrapnel came flying out, it was all at knee level for quite a distance. So the people close to that, that's where they were getting their injuries, that's what we saw here.

BLITZER: That's pretty scary. Thanks very much for that report. Eye-opening. Frightening indeed.

David Mattingly reporting for us. Appreciate it.

We're standing by now, not for one, but two live news conferences. We'll have live coverage. We just learned the Boston Police Department is going to be briefing all of us on the Boston -- the 19-year-old surviving suspect in this case. There's new information coming in. You're looking at live pictures from the Boston Police Department.

Also, that live news conference from Mississippi on the ricin- laced letters sent to President Obama and Senator Roger Wicker of Mississippi. That's Mississippi over there.

Boston and Mississippi, we're following live developments. Stay with us.

(COMMERCIAL BREAK)

BLITZER: We're standing by for live coverage of two news conferences. On the left, they're getting ready for a news conference here in Boston. Boston police holding a news conference to discuss what they described as the takedown of the 19-year-old bombing suspect who's in a hospital in Boston right now.

On the right, in Mississippi, they've got new information on those ricin-laced letters, those poisoned -- poisonous letter sent to the president and a U.S. senator among others. New information coming in on that. We'll have live coverage of both. Stand by.

Lots of news happening right now, right here in THE SITUATION ROOM.

Meanwhile, other news we're following, including some new claims that Syria is using chemical weapons against rebel forces.

Lisa Sylvester is monitoring that, some of the other top stories in THE SITUATION ROOM right now.

What do you have, Lisa?

LISA SYLVESTER, CNN CORRESPONDENT: Hi there, Wolf. Well, an Israeli defense official says in all likelihood the Syrian regime has used sarin gas on a number of occasions in the last few months resulting in victims suffering shrunken pupils or foaming from the mouth. Israel's Defense minister was less certain in saying the so- called red line hasn't been tested yet.

The Pentagon is assessing those reports but reiterated the use of such weapons would be entirely unacceptable.

Airline passengers are beginning to feel the effects of Washington's $85 billion in forced budget cuts. We are seeing a surge in flight delays at a number of airports as air traffic controllers take forced unpaid time off.

Furloughs kicked in Sunday for almost 50,000 employees. The FAA is required to cut $637 million from its budget by the end of September.

And at least another inch of rain is expected today in parts of an already flood-ravaged Midwest. At least six rivers in northern Illinois have surged to record levels and thousands of residents have been displaced. The governor declared 44 counties disaster areas. Missouri's governor has also declared a state of emergency.

And some momentary shock today after this A.P. tweet was posted, claiming, quote, "Two explosions in the White House and Barack Obama is injured." Well, it turns out that the A.P. account was hacked and the erroneous tweet was taken down a few minutes later.

The Dow plunged more than 100 points on the news but quickly regained its losses once the report was deemed false. You see that image right there. A group known as the Syrian Electronic Army behind previous media hacks has claimed responsibility for this tweet. An FBI spokesperson tells CNN the FBI is now investigating that hack -- Wolf.